Loss of visual acuity from refractive errors in a person's vision can be caused by conditions such as hyperopia and myopia. Some restoration of visual acuity can be provided by spectacles or contact lenses. Loss of visual acuity can also be caused by wavefront aberrations due to irregularities in the eye (e.g., in the cornea or in the natural crystalline lens). The lenses in spectacles, unlike contact lenses, have a stable orientation with respect to the eye, and can therefore be used to correct asymmetric wavefront aberration. Some “toric contact lenses” are ballasted to reduce rotation in the eye. Some irregularities in the cornea such as astigmatism can be partially corrected by a hard contact lens that conforms the cornea to the shape of the lens.